1. Field of the Invention
The present invention relates to a hydraulic controlling device of a working machine such as a hydraulic excavator.
2. Description of the Related Art
A related art will be described taking a hydraulic excavator as an example.
As shown in FIG. 5, in the hydraulic excavator, an upper rotating body 2 is mounted on a crawler-type lower traveling body 1 so as to be rotatable around a vertical axis. In addition, in the hydraulic excavator, a working (excavating) attachment 9 including a boom 3, an arm 4, a bucket 5, and cylinders 6, 7, and 8 for raising and lowering the boom, operating the arm, and operating the bucket, respectively, is mounted on the upper rotating body 2.
Left and right travel motors 10 and 11 that travel-drive the lower traveling body 1 and a rotation motor 12 that rotationally drives the upper rotating body 2 are provided (see FIG. 6).
FIG. 6 shows the entire structure of a hydraulic controlling device of the hydraulic excavator.
Hydraulic actuator groups are divided into a first group G1 and a second group G2. The first group G1 includes the right travel motor 11, the bucket cylinder 8, and the boom cylinder 6. The second group G2 includes the left travel motor 10, the rotation motor 12, and the arm cylinder 7.
With the travel motor 11 being defined as the uppermost stream side, the hydraulic actuators of the group G1 are connected in tandem with each other by a center bypass line CB1. Similarly, with the travel motor 10 being defined as the uppermost stream side, the hydraulic actuators of the group G2 are connected in tandem with each other by a center bypass line CB2. The hydraulic actuators other than the travel motors (hereunder referred to as the “working actuators”), that is, the hydraulic actuators 6, 7, 8 and 12 are connected parallel to oil pressure supply pipelines L1 and L2 that are provided separately from the center bypass lines CB1 and CB2. Reference characters T refer to tanks.
Hydraulic pilot control valves 13 to 18 for controlling the operations of the hydraulic actuators and remote control valves (not shown), serving as operating means for performing switching operations, are provided at the respective hydraulic actuators.
First and second pumps 19 and 20, serving as oil pressure supply sources for the hydraulic actuator groups, are provided. Oil discharged from the pump 19 and oil discharged from the pump 20 are supplied to the groups G1 and G2 through a hydraulic pilot straight travel valve 21.
The straight travel valve 21 is formed as a two-position, four-port switching valve having a neutral position X and a straight travel position Y that provide functions and having two pump ports P1 and P2 and two actuator ports a and b. The straight travel valve 21 is subjected to switching control by secondary pressure of a straight travel proportional valve 23, which is an electromagnetic proportional valve, based on a command from a controller 22.
Operation signals in accordance with operation amounts of the respective remote control valves (such as signals from pressure sensors that detect secondary pressures of the remote control valves) are input to the controller 22. When performing a single operational process in which a travel operation and working operation (operation of the working actuators 6, 7, 8, and 12) are performed separately, the straight travel valve 21 is at the illustrated neutral position X.
In this state, oil discharged from the first pump 19 flows through a path extending from P1 to b of the straight travel valve 21 and reaches the first group G1, and oil discharged from the second pump 20 is supplied directly to the second group G2. (This state will hereunder be called “first oil pressure supply state.”)
In a combined operational process in which the travel operation and the working operation are carried out at the same time, the neutral position X of the straight travel valve 21 is switched to the straight travel position Y.
In this state, the oil discharged from the first pump 19 flows through the oil pressure supply pipeline L1, and flows from a path extending from P1 to a of the straight travel valve 21 to the oil pressure supply pipeline L1 to be supplied to the hydraulic actuators 6, 7, 8, and 12 (which are the hydraulic actuators other than the travel motors 10 and 11). The oil discharged from the second pump 20 is distributed and supplied to the travel motors 10 and 11. (This state will hereunder be called “second oil pressure supply state.”)
Since, in the second oil pressure supply state, both of the motors 10 and 11 are driven by the common second pump 20, if they are travel-operated by the same amount, the same amount of oil is supplied to the travel motors 10 and 11, so that they rotate at the same speed. That is, straight travel ability is ensured.
In this case, since the amount of oil pressure supplied to the travel motors 10 and 11 becomes half that in the first oil pressure supply state, the speed is also halved (that is, is suddenly reduced). Therefore, a shock is generated.
To overcome this problem, a connection path 24, serving as means for reducing shock, is provided in the straight travel valve 21. When, in the second oil pressure supply state, pump lines of the pumps 19 and 20 are connected to each other by the connection path 24, a portion of the oil discharged from the first pump 19 is sent to a travel side (refer to Japanese Unexamined Patent Application Publication No. 2000-17693).
It is known that, when a controlling device has a structure in which a plurality of actuators are driven by a common pump, pressure interference that makes it difficult for oil to flow towards high operating pressure occurs.
According to this structure, in general, the amount of opening of the connection path 24 is fixed, and the amount of oil flowing through the connection path 24 is determined by a pump discharge amount and an operating pressure of each actuator.
In this case, if the pump discharge amount is sufficiently large, no conspicuous problems arise. However, when the rotational speed of an engine is low as during low idling, the pump discharge amount is small, thereby considerably reducing the amount of oil that is supplied to each actuator. Therefore, when carrying out a combined operational process in a state in which the engine is rotating at a low speed, the influence of the pressure interference is increased. As a result, work, such as raising a boom, requiring a load (operating pressure) that is higher than that required for traveling is improperly performed, that is, movement becomes extremely slow or no longer occurs).
Japanese Unexamined Patent Application Publication No. 2000-17693 discloses a technology in which the connection path is narrowed when the discharge pressures of the two pumps that are detected become equal to or greater than a certain value. However, this technology aims at preventing pressure interference that occurs due to a difference between the discharge pressures of the pumps. Therefore, the technology cannot be used to directly overcome pressure interference that occurs due to a change in the rotational speed of the engine.